Spectroscopic and X-Ray Diffraction Analysis

Characterizations of the metallacarbaborane species described in this review and in our earlier article (1) would not have been possible without modem tools of analysis. Of cmcial importance to this work are high-field multinuclear magnetic resonance spectroscopy and single-crystal X-ray diffraction analysis, two cornerstones of synthetic organometallic chemistry. It is therefore appropriate to highlight certain diagnostic features that are essential to the successful application of these techniques. 

The chemical shift ranges covering the majority of examples of the most important functional groups are shown in three figures in this section and are discussed further below. However, it is important to stress that both the chemical shifts and the profiles of the spectr peaks are powerful indicators of chemical stmcture. This is due largely to the presence in these molecules of at least nine quadmpolar boron nuclei ( B, I =, 80% B, I = j, 20%), which can be both a hindrance and an invaluable aid to stractural characterization. 

12-vertex metallacarbaborane complex having a 2,1,8-MC2B9 arrangement of atoms. 

The C-( H NMR spectrum is thus extremely powerful in determining the structures of metallacarbaboranes of the types described herein. Its value is further increased as the information it holds is often not provided in either H or B spectra. 

Very short relaxation times and the presence of 20% boron-10 (/ = ) combine to make the peaks in any B NMR spectrum broad. These factors are exacerbated in metallacarbaborane complexes such as those 

Diagnostic chemical shift ranges in the1H NMR spectra from metallacarbaborane complexes of the types described herein. 

5 Docosahedral MC2Bl0 cages contain a puckered 6-membered BBBCBC ring that flexes, in solutions at ambient temperature, on the NMR time scale (7). Spectra of the static structure, in which the carbon vertices necessarily have different environments with respect to the metal center, may generally be obtained by cooling to —20°C. 

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The crystallization and X-ray diffraction analysis of the reaction center from the bacterium Rb. sphaeroides R-26 has revealed the three-dimensional structure of the protein and bound cofactors to atomic resolution (Allen et al., 1986 Allen et al., 1987 Yeates et al., 1987 Yeates et al., 1988 Allen et al., 1988 Komiya et al., 1988 Chang et al., 1991 El-Kabbani et al., 1991). With this information available, it is of interest to ask how the spectroscopic properties of the reaction center correlate with the structural features. Ultimately this correlation will allow an elucidation of the molecular details that control the spectral features and relate to the primary photochemical events carried out by the reaction center. Polarized light absorption is one technique for correlating the spectroscopic features with its molecular structure (Breton, 1985). In order to make the correlation more precise, it is distinctly advantageous to carry out the spectroscopic experiments directly on the crystalline samples used in the X-ray diffraction analyses. In this way the clearest link between the structure of the complex and its photochemical properties will emerge. 

Besides synthesis, current basic research on conducting polymers is concentrated on structural analysis. Structural parameters — e.g. regularity and homogeneity of chain structures, but also chain length — play an important role in our understanding of the properties of such materials. Research on electropolymerized polymers has concentrated on polypyrrole and polythiophene in particular and, more recently, on polyaniline as well, while of the chemically produced materials polyacetylene stih attracts greatest interest. Spectroscopic methods have proved particularly suitable for characterizing structural properties These comprise surface techniques such as XPS, AES or ATR, on the one hand, and the usual methods of structural analysis, such as NMR, ESR and X-ray diffraction techniques, on the other hand. 

These spectroscopic data support a square-pyramidal structure in solution with the phosphines mutually trans disposed, the chloride and the carbonyl group occupying the basal sites, and the hydride ligand located at the apex. This structure fully agrees with that found in the solid state, by X-ray diffraction analysis for the related compound OsHCl(CO)(PCy3)218, and for ab initio DFT (Becke 3LYP) methods for the model complex OsHC1(CO)(PH3)2.19... 

The structures of the bryostatins were determined by a combination of singlecrystal X-ray diffraction analysis and/or a series of detailed spectroscopic analyses. 

Dendrimers have precise compositional and constitutional aspects, but they can exhibit many possible conformations. Thus, they lack long-range order in the condensed phase, which makes it inappropriate to characterize the molecular-level structure of dendrimers by X-ray diffraction analysis. However, there have been many studies performed using indirect spectroscopic methods to characterize dendrimer structures, such as studies using photophysical and photochemical probes by UV-Vis and fluorescence spectroscopy, as well as studies using spin probes by EPR spectroscopy. 

An interesting observation was made upon isolation and characterization of the amplified product. NMR spectroscopic data together with supporting X-ray diffraction analysis clearly proved the compound not to he iminolactone (56), but rather lactam (57). Further mechanistic studies are yet to he carried out for the proposed rearrangement, hut a few related systems are reported [46 8]. 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

Croomine is a major alkaloid of the roots and rhizomes of Croomia heterosepala. Its structure (10) has been settled by its chemical reactions and spectroscopic properties, and confirmed by X-ray diffraction analysis of its methiodide. On mild oxidation (dehydrogenation) with silver oxide, the alkaloid affords the corresponding pyrrole.8... 

The relative configuration of cynometrine and isocynometrine were determined by X-ray diffraction analysis. The absolute configuration of the phenyl carbinol chiral center was determined by the method of Horeau and Kagan (154). From UV spectroscopic data and the rotations of the different alkaloids and their derivatives, the steric structures 94-102 have been deduced as depicted in the illustrations (155). 

Several guanidine-modified tetrapeptides have been isolated from marine invertebrates, in particular marine sponges. Nazumamide A (39) has been isolated from the sponge Theondla sp. and identified by analysis of spectroscopic data [60] as well as by X-ray diffraction analysis of a nazumamide A-human thrombin complex [61[. Nazumamide A has been synthesized by conventional peptide synthesis [62[. A series of nazumamide derivatives have been prepared via combinatorial synthesis [63[. 

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